Date: Sat Oct 28 2000 - 14:06:57 PDT
Dilpreet and Thomas:
The original question:
I was wondering if the selection of the decoupling capacitor of a circuit
is MAINLY governed by the edge rates of the switching signals or by the
frequency of their switching.
Both. As pointed out earlier, Fourier analysis would indicate the frequency
content of the signal and the device self-resonant frequency (clearly
impacted by the device packaging AND the PCB layout). Both will impact the
magnitude of decoupling achieved.
For a symmetrical trapezoidal waveform (idealized, of course), 99% of the
waveform energy is contained in the frequency band from the fundamental
frequency to 1/PIxTr, where Tr (risetime) = Tf (falltime). For fundamental
frequencies >20 MHz, the series inductance (determined by the device and the
packaging traces and loop area) establishes the decoupling impedance. Your
should derive your needs from this point forward as to the number of
capacitors, packaging techniques, etc.
Michael L. Conn
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